This application relates to battery power packs of the type used for jump-starting automotive vehicles and, in particular, to lead-acid battery packs of the thin metal film type. The application relates in particular to charge maintenance in such battery power packs.
It is known to provide portable battery power packs for providing DC power in emergency situations and/or remote locations, such as for the purpose of jump-starting automotive vehicle engines, the batteries of which have become discharged for some reason. Such battery power packs typically include lead-acid batteries of a capacity to provide DC voltage and current sufficient for starting an automotive vehicle engine. Battery powerpacks are typically rechargeable and, since lead-acid batteries can become irreparably damaged if their voltage drops below a predetermined value, it is desirable that they be recharged after each use. A simple way of recharging such a battery power pack is to leave it connected to the battery of the vehicle being jump-started so that, after the vehicle engine starts, the battery power pack can be recharged by the vehicle charging system (alternator or generator). Prior battery packs have also been provided with auxiliary charger ports so that they can be connected to free-standing chargers when not connected to the vehicle.
However, lead-acid battery packs can also undergo a significant amount of self-discharge during extended periods of non-use, such as during storage, transportation or the like. Accordingly, it is also known to provide a lead-acid battery power pack with a built-in set of alkaline batteries to provide a "keeper" charge current through a diode and a resistance to make up for self discharge while "on the shelf." However, such built-in keeper charge systems add to the size, weight, complexity and overall cost of the battery power pack. Thus, the designer of the battery power pack is faced with a decision to reduce the size of the keeper batteries to minimize size, weight, cost and the like, while also minimizing the length of time that the charge of the battery power pack can be maintained, or increasing the size of the keeper batteries to lengthened the time that the charge can be maintained on the battery power pack, at the expense of added size, cost, weight and the like.